1. Field of the Invention
The present invention relates to a cleaning evaluation method for a substrate, and more particularly to a method for evaluating a surface cleanliness of a substrate which has been cleaned after being polished in a semiconductor fabrication process.
2. Description of the Related Art
As semiconductor devices have become more highly integrated in recently years, circuit interconnects have become finer and distances between those interconnects have also become smaller. In order to enable an interconnect structure to be fine, it is necessary to flatten a surface of a semiconductor wafer having the interconnect structure formed thereon. Specifically, if a surface of the semiconductor wafer has irregularities whose heights are larger than a depth of focus of a stepper, high resolution cannot be obtained in an exposure process, and hence a fine interconnect pattern cannot be formed. For this reason, it has been practiced to polish a surface of a semiconductor wafer to flatten the surface with use of a polishing apparatus that performs a chemical mechanical polishing (CMP).
As shown in FIG. 7, this type of polishing apparatus (CMP apparatus) comprises a polishing table 302 having a polishing pad (e.g., a polishing cloth) 300 attached to an upper surface thereof, and a top ring 304 for holding a semiconductor wafer W, to be polished, in such a state that a surface to be polished faces the polishing table 302. An upper surface of the polishing pad 300 serves as a polishing surface 301. In this polishing apparatus, the polishing table 302 and the top ring 304 are independently rotated, and the semiconductor wafer W is pressed against the polishing surface 301 at a predetermined pressure by the top ring 304 while a polishing liquid (slurry) is supplied from a polishing liquid supply nozzle 306, disposed above the polishing table 302, onto the polishing surface 301.
The polishing liquid supplied from the polishing liquid supply nozzle 306 comprises, for example, alkaline solution in which fine abrasive grains, such as silica particles, are suspended. Thus, the semiconductor wafer W is chemically and mechanically polished to a flat mirror finish by a combination of a chemical polishing action of alkali and a mechanical polishing action of the abrasive grains. In recent years, a fixed abrasive comprising abrasive grains, such as cerium oxide (CeO2), fixed together by a binder has also been used instead of the polishing pad (polishing cloth).
After the polishing apparatus performs a polishing process, a polishing capability of the polishing surface 301 of the polishing pad 300 is lowered. Therefore, in order to recover the polishing capability of the polishing surface 301, a dresser 308 having a dressing member 310 on a lower surface thereof is provided. This dresser 308 dresses the polishing surface 301 of the polishing pad 300 when replacing the polished semiconductor wafer W or at other timings. The dressing process is carried out as follows: The dresser 308 and the polishing table 302 are rotated independently while a dressing liquid, such as pure water, is supplied onto the polishing surface 301. Then, the dressing member 310 of the dresser 308 is pressed against the polishing surface 301, whereby the polishing liquid and shavings of polished material (e.g., Cu as interconnect material), each remaining on the polishing surface 301, are removed. At the same time, the polishing surface 301 is flattened and dressed, whereby the polishing surface 301 is regenerated. This dressing is also called conditioning.
After the polishing process is finished, the semiconductor wafer W is transferred to a non-illustrated cleaning device, so that the polishing liquid and the shavings of the polished material attached to the surface of the semiconductor wafer W are removed by the cleaning device. Thereafter, the semiconductor wafer W is transferred to a drying device, and is then dried by a spin-drying process or the like.
The abrasive grains contained in the polishing liquid (hereinafter referred to as particles), and the shavings of metal (i.e., interconnect material) and heavy metal component contained in the polishing liquid (hereinafter referred to as heavy metal) are known as a contaminant that has an adverse effect on the subsequent various processes. If the particles and the heavy metal remain on the surface of the semiconductor wafer, a subsequent film-forming process and an exposure process cannot be performed normally, and hence a pattern defect, such as a short circuit, is caused. Therefore, in the above-mentioned cleaning process using the cleaning device, it is required to sufficiently remove the contaminants from the semiconductor wafer in order to achieve a high surface cleanliness of the semiconductor wafer.
In order to check whether or not the contaminants have been sufficiently removed from a semiconductor wafer, a cleaning evaluation apparatus has been used for evaluating a surface cleanliness of a semiconductor wafer. As this type of cleaning evaluation apparatus, an optical inspection instrument utilizing scattered light of a laser light and an X-ray fluorescence analyzer utilizing fluorescence X-ray emitted from a specimen have been known. These cleaning evaluation apparatuses judge whether or not the contaminants, such as the particles, are removed from the surface of the semiconductor wafer to a degree less than or equal to an allowed value.
A semiconductor wafer on which the cleaning evaluation is to be performed is originally a substrate which has circuit patterns constituted by a metal (e.g., Cu) as an interconnect material, an insulating film (e.g., TEOS film or low-k film), and the like (hereinafter this substrate will be referred to as a patterned substrate). However, such a patterned substrate is rarely used in the cleaning evaluation process. In practice, a semiconductor wafer having only a TEOS film formed on a surface thereof is often used for the cleaning evaluation. This is because a technique for quickly and accurately evaluating a surface cleanliness of the patterned substrate has not been established yet.
Hereinafter, a conventional cleaning evaluation process for a semiconductor wafer will be described. First, a dummy substrate, which has a metal film (e.g., a Cu film) as an interconnect material formed on a surface thereof, is polished by a polishing apparatus. The dummy substrate is a substrate whose surface is covered with a metal film of the same kind as an interconnect material used in pattern formation. After the dummy substrate is polished, the polishing surface of the polishing pad is dressed (conditioned), so that shavings of the metal film and the polishing liquid are removed from the polishing surface. The polished dummy substrate is transferred to the cleaning device and then cleaned, and is further transferred to the drying device and then dried. In this manner, a plurality of (e.g., three) dummy substrates are polished by the polishing apparatus. Thereafter, a semiconductor wafer serving as an object of the cleaning evaluation (hereinafter, referred to as a monitor substrate) is polished by the polishing apparatus. The monitor substrate is a substrate having an oxide film formed on a surface thereof. The monitor substrate which has been polished is transferred to the cleaning device and then cleaned, and is further transferred to the drying device and then dried. The monitor substrate, which has been processed in this manner, is transferred from the polishing apparatus to the cleaning evaluation apparatus. This cleaning evaluation apparatus measures an amount of contaminants, such as particles and heavy metal, remaining on the oxide film of the monitor substrate, and judges whether or not these contaminants are removed to a degree less than or equal to an allowed value.
However, the above-mentioned conventional cleaning evaluation process includes the following problems:
First, if the polishing surface of the polishing pad is dressed (conditioned) before the monitor substrate is polished, the shavings of the metal film of the dummy substrate remaining on the polishing surface (hereinafter such shavings will be referred to as metal contaminants) are removed to a certain degree, and hence an amount of metal contaminants attached to the surface of the monitor substrate that is subsequently polished is reduced. Accordingly, a state of existence of the metal contaminants on the surface of the monitor substrate is different from that of the metal contaminants in the case of the polished patterned substrate. Specifically, after the patterned substrate is polished, the shavings of the metal constituting the circuit pattern are generally present on the surface of the patterned substrate. However, if the polishing surface is dressed before the monitor substrate having only the oxide film is polished, this monitor substrate is polished in the presence of a small amount of metal contaminants. Accordingly, a different state of existence of the metal contaminants is created on the polished monitor substrate compared with the patterned substrate. As a result, the cleaning evaluation apparatus cannot perform an accurate cleaning evaluation.
Second, it is relatively difficult to obtain a low-k film which is expected to serve as an insulating film for Cu interconnects, and hence a substrate having an oxide film (e.g., a TEOS film) is used as the monitor substrate, as described above. However, there is a difference in the remaining amount of the metal contaminants between the oxide film and the low-k film even when the substrate is polished under the same condition. FIG. 8 is a graph illustrating amounts of the metal contaminants remaining on the low-k film and the TEOS film. In FIG. 8, the ordinate represents the number of atoms of the metal contaminants per 1 cm2, and reference signs A through O arranged along an axis of the abscissa represent samples. As shown in FIG. 8, there is a large difference between the amount of the metal contaminants remaining on the TEOS film and the amount of the metal contaminants remaining on the low-k film. Accordingly, if the monitor substrate having an oxide film, such as a TEOS film, is used when intending to evaluate a cleanliness of the low-k film, an accurate result of the cleaning evaluation cannot be obtained.